An optical filter is a device for selectively allowing light having a particular wavelength to transmit. In addition, the optical filter is a device incorporated into optical transmission devices and the like and therefore, is demanded to be reduced in size according to the trends of the times.
A wavelength selectable switch used in building an optical multiplexing communication system also functions as the optical filter. The wavelength selectable switch is a device that selects signal light having a particular wavelength from a wavelength-multiplexed input signal using a diffraction grating, and outputs the selected signal light through one of a plurality of output fibers. Accordingly, by using only one of the plurality of output fibers, the wavelength selectable switch functions as the optical filter.
Related arts are disclosed in, for example, Japanese Laid-open Patent Publication Nos. 2011-65023 and 2009-9073, and Mona Mayeh and Faramarz Farahi, “Tailoring Gaussian Laser Beam Shape Through Controlled Etching of Single-Mode and Multimode Fibers: Simulation and Experimental Studies”, IEEE SENSORS JOURNAL, VOL. 12, NO. 1, pp. 168-173, JANUARY 2012.
For the optical filter, the relation between the transmittance and the wavelength (hereinafter referred to as transmission characteristic) is an important characteristic. The preferred transmission characteristic of the optical filter is a filtering characteristic that the transmittance is high in a certain wavelength range (for example, 1 dB band), and suddenly decreases on the both sides of the wavelength range.
For example, using a general variable-wavelength filter used in an optical module as an example of the optical filter, it is feasible to change the Gaussian filtering characteristic into a desired filtering characteristic (that is, filtering characteristic that the transmittance suddenly decreases on both sides of a wavelength range) by providing a slit plate between the diffraction grating of the wavelength selectable switch and the output fiber. However, the slit plate makes downsizing of the variable-wavelength filter difficult. As described above, the downsizing of the optical filter including the variable-wavelength filter follows the trends of the times, such hard-to-downsize optical filter is unpractical.
In consideration of such situation, it is desired to realize an optical filter having the above-mentioned filtering characteristic without increasing the size.